Libraries and retail stores often use electronic article surveillance (EAS) systems to protect articles such as books or prerecorded magnetic video and audio cassettes from unauthorized removal. Dual-status magnetic EAS markers are a good choice for this application, but the relatively large magnetic fields required to deactivate the markers is more than sufficient to degrade the prerecorded magnetic signals on audio or video cassettes to a degree that is audibly or visually perceptible by human beings. Such effects, including print through and partial erasure, are highly undesirable.
Dual-status magnetic EAS markers typically comprise a layer or strip of high permeability magnetic material and one or more segments or layers of remanently magnetizable members adjacent the high permeability material. When the remanently magnetizable members are in a demagnetized state, they do not magnetically interact with the high permeability material, and can be reversibly driven between oppositely directed saturated magnetized states in response to the interrogation field from an EAS detection system, providing a detectable signal in the system. When the remanently magnetizable members are in the proper remanently magnetized state, they provide stronger magnetic fields to the high permeability material than the interrogation fields, and retain it in a constant magnetized state and prevent it from being reversibly driven between oppositely directed saturated magnetized states and providing a detectable signal. Thus, the dual-status markers are deactivated by remanently magnetizing the remanently magnetizable members.
The deactivating process typically involves properly orienting the marker and then passing it through a magnetic field with deactivating components along the direction of translation. Deactivating devices preferably provide magnetic fields which are constant in time, spatially uniform in the transverse direction over the extent of the deactivator, and spatially varying in the other two directions. The longitudinal component of the magnetic field at the surface contacting the marker should be at least 1.4 times the (200 to 350 Oe.) coercive force of the remanently magnetizable marker material to assure adequate remanent magnetization. However, such a magnetic field will result in undesirable levels of signal degradation of the recorded signal on a typical audio cassette (having a typical coercive force of about 300 Oe.) or even a video cassette (having a typical coercive force from 550-1300 Oe.). Self-demagnetizing fields associated with a cassettes own recorded magnetization patterns, as well as magnetic fields from the recorded patterns on adjacent layers of the tape, also affect the recorded signal. When the magnetic fields from the deactivating device are superimposed on the fields originating from the magnetic media, they act as an effective bias in promoting self-demagnetization and the imaging or printing of the magnetic field patterns from adjacent layers. For example, it has been found that for the prerecorded magnetic tape in audio cassettes, a magnetic field from the deactivating device as low as 100 Oe will result in levels of signal degradation that are perceptible by humans.
To avoid such deleterious effects on prerecorded magnetic media, it is also known to provide apparatus in which a steady-state field is produced which rapidly decreases in intensity with increased distance from the apparatus. Thus, such an apparatus improves the likelihood of magnetizing the higher-coercive force sections of a marker brought close thereto without interfering with the magnetic signals recorded on tapes within a cassette to which the marker is affixed. For example, the apparatus described in U.S. Pat. No. 4,499,444 to Heltemes et at. has generally been found to be satisfactory so long as it is used with markers of a single type, and whose magnetizable components all have a coercive force within a given range, such that the field intensity at the working surface of the apparatus is controlled to appropriately magnetize those components while not adversely affecting magnetically sensitive articles. Conversely, it has been found that when the apparatus is used with markers nominally of the same type, but in which the value of the coercive force varies over a relatively wide range of allowed values, certain conditions may cause unsatisfactory results.
To prevent adverse effects on magnetically sensitive articles with which the markers are desirably used, the field intensity at some distance from the working surface of the apparatus at which such magnetically sensitive articles are to be located must be below certain design limits. However, a practical apparatus desirably has an effective operable range extending a short distance above the surface within which all allowed materials must become magnetized. Some markers having coercive forces near the highest allowed value and positioned near the outer edge of the allowed range, i.e., in the weakest fields, may not become sufficiently magnetized. And, since known deactivating devices include a reverse directed back field, which is particularly strong near the surface of the apparatus, such back fields may be sufficient to reduce the magnetization state in markers near the surface and having coercive forces near the lowest allowed value. Such reduced magnetization levels could, in turn, inadequately bias the low coercive force, high permeability material of the marker, such that the response of the marker would be inadequately altered. Such effects are further compounded if markers of significantly different types, each having magnetizable materials having coercive forces in significantly different ranges are used with the same apparatus.
The widely varying geometries presented by articles to which markers are commonly attached also present special problems. For example, protruding or recessed portions of many articles prevent placement of the article flat against a deactivating surface. This results in parts of the article being located farther away from the deactivating surface such that a marker placed there may not be properly deactivated. The typical audio and video cassettes are examples of such articles which present geometries where this problem commonly arises.
The different types of markers used in EAS systems have magnetizable elements in a range of coercive forces. For example, one type of marker has a magnetizable element with a coercive force in the range of 24,000-28,000 A/m (300 to 350 oersteds), a second type has a magnetizable element with a coercive force in the range of 14,400-18,400 A/m (180 to 230 oersteds), and a third type has a magnetizable element with a coercive force in the range of 4,800-7,200 A/m (60-90 oersteds). Such markers may, for example, be type QT Quadratag.TM., Type WH-0117 Whispertape.TM. and type QTN Quadratag.TM. markers, respectively, all of which are sold by Minnesota Mining and Manufacturing Company (3M), St. Paul, Minn.
In addition to the signal degradation, currently available deactivating devices also suffer from ergonomic problems. Often a person is required to repetitively deactivate markers on multiple articles over an extended period of time. The repetitive picking up, twisting, translating, and setting down of articles required to deactivate the markers is the type of repetitive motion associated with hand, arm, and wrist fatigue, and, in the worst case, Carpal Tunnel Syndrome. Thus, an ergomonic housing design which alleviates this problem would be highly desirable.
It would therefore be desirable to have a deactivating apparatus whose magnetic field strength decreased rapidly away from the magnet assembly, which is adapted for deactivation of markers on both audio and video cassettes such that any signal degradation of the associated prerecorded magnetic media is not audibly or visually perceptible to human beings. Other features which would be desirable in a deactivating device include a low profile, ergonomically designed housing such that adverse physical effects on a human operator and interference with other checkout procedures are minimized, and which requires fewer components and less material than presently known devices.